FRP vessel leaking or bulging — causes and what to do
Short answer: Leaks are almost always at the top thread, usually from over-tightening or a worn O-ring. Bulging is more serious — it means the vessel has been over-pressured, and a bulged vessel should be replaced, not repaired.
Identify the problem first
| What you see | Likely cause | Repairable? |
|---|---|---|
| Leaking at the top thread | O-ring, or over-tightening | Usually yes |
| Leaking at the side flange | Gasket | Yes |
| Weeping from the body | Crack in the wall | No — replace |
| Visible bulge | Over-pressure | No — replace |
| Dented inward | Vacuum collapse | No — replace |
| Media in the treated water | Broken lateral (not a vessel fault) | Yes, internals |
Cause 1: Leaking at the top thread
The most common complaint, and usually the simplest.
Over-tightening
People tighten the valve hard in the belief it will seal better. In fact it distorts the O-ring so it stops sealing, and it can crack the thread.
The correct approach: hand tight, then a little more. The seal is made by the O-ring, not by force.
Worn O-ring
Inspect the O-ring. If it is flattened, cut or hardened, replace it. Apply silicone grease — never ordinary oil or grease, which attacks rubber.
Dirt in the groove
A single grain of grit under the O-ring will cause a leak. Clean the groove thoroughly before refitting.
Cause 2: Bulging from over-pressure
Standard FRP vessels are rated to around 10 kg/cm². Beyond that the wall begins to deform.
Why over-pressure happens
- Pump pressure higher than the vessel rating
- Outlet valve closed while the pump runs — pressure builds with nowhere to go
- Water hammer — a valve slamming shut sends a pressure surge
- No pressure relief in the system
What to do
Replace the vessel. A bulged FRP vessel cannot be repaired — its structure is already compromised and it may fail suddenly.
And find the cause, or the new vessel will suffer the same fate. Fit a high pressure switch (₹546) so the pump stops before pressure becomes dangerous.
Cause 3: Vacuum collapse
This is the reverse problem, and it surprises people. If a vessel drains and no air can enter, the pressure inside falls below atmospheric and the vessel is crushed inward.
When it happens
- The supply stops while the outlet is still draining
- A pump draws from the vessel with the inlet closed
- The system is drained down for maintenance
Prevention
Fit a vacuum breaker / air release valve 25NB — only ₹210. It lets air in when pressure falls, so a vacuum can never form.
Cause 4: Crack in the body
Usually caused by impact, prolonged sunlight, or age. A cracked vessel cannot be repaired — FRP repairs do not hold reliably under pressure.
Warning signs before a crack appears:
- Colour fading or a chalky surface
- Fine surface crazing
- Fibres becoming visible
A vessel showing these should be planned for replacement.
What is safe to repair
| Problem | Action |
|---|---|
| O-ring leak | Replace the O-ring, apply silicone grease |
| Flange gasket leak | Replace the gasket |
| Minor thread damage | Sometimes an adaptor helps — ask us |
| Broken lateral | Replace the internals |
| Bulge, dent or crack | Replace the vessel |
Five ways to prevent damage
- Fit a vacuum breaker — ₹210, prevents collapse
- Fit a high pressure switch — ₹546, stops the pump before over-pressure
- Hand tighten only — never force the valve on
- Keep it out of direct sunlight — or shade it
- Open and close valves slowly — avoiding water hammer
All of that protection costs under ₹800, against the cost of a vessel, its media and the downtime.
Safety when working on a vessel
- Stop the pump
- Close the inlet valve
- Release pressure through the drain or air release valve
- Confirm the pressure gauge reads zero
- Only then open the valve or manway
When to replace rather than repair
If you are weighing repair against replacement, apply this test: anything affecting the vessel wall means replacement. Bulges, dents and cracks are all structural. Only seals, gaskets and internals are genuinely repairable.
For replacement options see our vessel size chart.
Vessel problem?
Send us the vessel size and what you are seeing, and we will tell you whether it can be repaired or needs replacing.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
Why is my FRP vessel leaking at the top?
Almost always the O-ring, either worn or badly seated, or the valve having been over-tightened. Tightening harder makes it worse because it distorts the O-ring. Replace the O-ring, apply silicone grease and hand tighten only.
Can a bulged FRP vessel be repaired?
No. A bulge means the vessel has been over-pressured and its structure is compromised, so it can fail suddenly. It must be replaced, and the cause of the over-pressure must also be found and fixed.
What causes a vessel to collapse inward?
A vacuum. If the vessel drains and no air can enter, internal pressure falls below atmospheric and the vessel is crushed inward. A vacuum breaker or air release valve prevents this and costs very little.
How do I prevent over-pressure damage?
Fit a high pressure switch so the pump stops before pressure becomes dangerous, and open and close valves slowly to avoid water hammer. Never run the pump with the outlet valve closed.
What can safely be repaired on a vessel?
O-rings, flange gaskets and internal distribution components can all be replaced. Anything affecting the vessel wall itself, such as a bulge, dent or crack, means the vessel has to be replaced.